The present invention relates generally to surface acoustic wave devices, and in particular to a surface wave termination for such devices.
Surface acoustic wave (SAW) devices have been employed for a wide variety of signal processing functions, and the basic technology is well known in the art.
In elementary form, a surface wave transducer comprises a pair of parallel, spaced metal electrodes disposed on a piezoelectric substrate. When an alternating electrical potential is applied to the electrodes, an alternating electric field is generated that causes localized vibrations in the substrate material. These vibrations give rise to acoustic waves, which propagate along the surface of the substrate in a well defined path orthogonal to the electrodes, and may be detected at any point along the path by a second, receiving transducer. Typically, both the transmitting, or launching, transducer and the receiving transducer are formed from interdigitated comb-like multi-electrode elements, the electrical characteristics of the transducer being determined by the number, spacing, and dimensional configuration of the electrodes in each element. The electrodes typically are formed from deposited metal films and have a thickness less than 10% of the acoustic wavelength to minimize mass loading.
Transducers of the type just described are bidirectionally responsive, i.e., they launch surface waves in opposite directions simultaneously and receive waves traveling in either direction. This is a significant problem in most SAW devices because in addition to responding to surface waves traveling directly from one transducer to the other, the transducers respond to surface waves reflected from the ends of the substrate. These reflected surface waves produce unwanted signals that distort the main, desired signal, adversely affecting the performance of the SAW device. A widely practiced method of suppressing the reflected waves is to apply an absorbent material, typically "black wax", to the edges of the substrate behind each transducer. Black wax is applied in the form of a solvent solution. After evaporation of the solvent, a hard, brittle wax deposit remains. While black wax minimizes reflections from the substrate edges fairly effectively, surface waves are reflected from the front edges of the hard wax deposits. These reflections are of reduced amplitude, but still degrade SAW device performance.
A phasing method for eliminating the effect of substrate and reflections is disclosed in U.S. Pat. No. 3,955,159. No absorbent material is used; instead, the transducer is constructed in such a way that end-reflected waves are cancelled out at the receiving transducer. For complete cancellation, however, the transmitting and receiving transducers must be perfectly aligned relative to each other and to the adjacent ends of the substrate, which must be perfectly smooth and orthogonal to the wave path. In addition, the phasing method is limited in application to relatively narrow frequency range devices.
Accordingly, a general object of the present invention is to provide means for substantially eliminating output distortion or spurious responses in SAW devices caused by reflections of surface acoustic waves from the ends of the device's substrate. A more specific object is to provide a surface wave termination system that inhibits such reflections.
Another object of the invention is to provide means for inhibiting surface wave reflections from the end of a SAW device substrate without the disadvantages associated with prior art methods.
Still another object of the invention is to provide a SAW device surface wave termination system that employs readily available materials and standard application techniques.
Briefly stated, the invention contemplates a two-stage surface wave termination system that is applied to a SAW device substrate intermediate as interdigitated transducer provided thereon and the end of the substrate nearest the transducer. One stage comprises a wafer of an acoustically lossy polymeric material mechanically coupled, as by bonding, to the substrate surface. The edge of the wafer nearest the transducer preferably is skewed relative to the main path of surface acoustic wave propagation on the substrate. The second stage of the system comprises a band of elastomeric material secured to the substrate surface along the edge of the wafer nearest the transducer. In operation, surface acoustic waves propagated toward the end of the substrate are effectively absorbed or otherwise reduced to an undetectable level by the termination system.
Additional objects, features and advantages of the present invention will become apparent as the following detailed description is read in conjunction with the accompanying drawings.